An apparatus for treating contaminated fluid has a UV lamp within an elongate housing, typically rectangular shaped. Deflector baffles within the housing create a meandering pathway perpendicular to the lamp for exposing the fluid to the UV light along the entire length of the pathway. Fins create multiple channels in the pathway to substantially increase internal surface area contact with the fluid. A photocatalytic coating on the baffles, fins and internal surfaces of the housing is maintained within a pre-set radial distance, preferably within about 75 mm, from the lamp for optimal creation of a photocatalytic reactant. The fluid flowing along the pathway, including channels, is also maintained in close proximity to the lamp and has adequate time for exposure to the ultraviolet light and photocatalytic reactant for treatment before exiting the housing. Baffles and fins are removably positioned within the housing for convenient maintenance, pathway length alteration or accommodating fluid volume flow adjustment, with little or no housing re-sizing. This apparatus is an affordable and compact environmental protection device capable of “air quality refinement”, especially for building ventilation systems, by mitigating the spread of harmful pathogens and/or toxic chemicals.